1. Field of the Invention
This invention relates to a method for determining the quantity of a coating applied to a material web, in particular a fibrous web. Also, the invention relates to a method for controlling the quantity of a coating applied to a material web. In addition, the invention relates to an apparatus for applying a coating to a material web.
2. Description of the Related Art
Goods interact with the environment as a rule only with their surface. Hence, products such as foils or papers and the like are often coated nowadays.
Papers are finished in this case in a coating process in order to influence their functionality for example. Thermal papers and copy papers can be cited as examples in this connection. However, more and more papers are being coated in order to improve the printability of the paper. For example, papers for printing newspapers and magazines are coated in a coating process, thus giving rise to papers suitable for mass printing such as LWC paper for example.
The quantity of coating applied is decisive in this case for the quality of such papers. Hence, a certain minimum coating quantity on the paper for example must be observed. However, for the cost-effective and hence economical production of such papers it is also imperative for the coating quantity not to exceed certain maximum limits.
In the prior art, various techniques for determining the coating quantity, also referred to as coating weight, are in use today.
For example, the coating weight can be determined from the difference between the gsm substance before and after the coating process.
However, with the method mentioned it is possible to determine only the total coating weight, meaning the sum of the coating weight resulting from the coating of the top side and the bottom side. This is a disadvantage in particular when the coating is applied simultaneously to both sides. If the coating is applied first to the one side of the web and subsequently to the other side of the web, then the coating weight has to be measured after each application pass, which requires two measuring devices.
Mention must also be made of spectroscopic methods with which a reference signal and a measurement signal of a principal component are compared spectrally with each other, the underlying physical approach being based on Lambert Beer's law. The principal component used is kaolin, latex or CaCO3 for example. Using the spectroscopic method it is possible as a rule to determine the coating weight separately for each side.
Such a method is described in EP 0 882 945 for example.
A disadvantage of these methods is, on the one hand, that the concentration of the principal component has to be sufficiently high and, on the other hand, that Lambert Beer's law presupposes a homogeneous and non-diffusing sample. In reality, however, there are diffusing particles in the sample to be measured, so the accuracy of this method is often unsatisfactory.
Methods which try to circumvent the above mentioned disadvantages use high-resolution laser spectrometry. These methods are complex and cost-intensive however. Furthermore, high-resolution laser spectrometers are sensitive and unsuitable for use in adverse conditions such as exist in the area of a paper machine.
What is needed in the art is a method of the type initially referred to, with which the described disadvantages no longer arise.